It is known in the prior art to treat pulverized granular dusty particles in fluid beds with a gaseous fluid. The fluid is blown through a perforated distributor bottom in a vertical direction upwardly across the fluid bed. The particles are converted into a highly fluid mass and maintained in suspension while they are exposed to intensive contact with the treatment gas. These fluid bed processes of the prior art have essential disadvantages. If big and small particles are present, it is difficult to maintain a gas velocity which is suitable to keep all particles in suspension. Small particles can be entrained by the fluidizing gas whereas large particles are not even elevated. The second disadvantage is the comparatively low upper limit of the gas velocity. This requires large equipment, in order to keep the allowable maximum gas velocity below its upper limit at a given feed rate of the goods.
To overcome these disadvantages, flash dryers have been proposed as described in United Kingdom Pat. No. 981,750 and Swiss Pat. No. 520,380. These devices, however, are restricted to high gas velocities, because particles are held in suspension by the rotating gas flow only. This produces high pressure losses and, therefore, high production costs. The high rotational speed of the gas flow can have another disadvantage, an undesired attrition of the particles. A further drawback is involved by the fact that deposits are likely to be formed despite the high rotational speed of the gas. This occurs more readily if the particles to be treated are sticky. These deposits impede the flow and additionally contribute to further deposits.
Another drawback is the fact that various particles of the goods have an undefined path of flight, causing various retention times and, therefore, various treatment periods. In order to obtain a homogeneous treatment period, several apparatuses must be arranged in series which is expensive and space consuming.
Furthermore, the cleaning effort and the pressure difference are much greater when these multi-stage units are utilized. Another drawback of this system is the necessity of installing separators, if counter-directional flow is desired, which represents a source of pressure differentials and requires spacious devices and installations.
It is therefore the object of the present invention to overcome these disadvantages by the arrangement of the fluid bed casing opposite to the discharge nozzle, combined within an annulus, which is concentric and peripherous, for the entry of a second treatment gas and the discharge of the goods. The invented process provides a conversion of the rising gases into a circular fluid bed in an upwardly rising flow of treatment gas. The fluid bed is simultaneously accelerated by the treatment gas flow into the rotating movement, the treatment gas flow being introduced from a tangential direction. The treatment gas is discharged above the fluid bed, whereas the goods are removed from the bed in an opposite direction to the upward treatment gas flow. The goods are kept in suspension by the treatment gas flow rising through the annulus. The flow rate required for this purpose is only a fraction of the total treatment gas flow rate. The greater portion of the treatment gas is introduced in tangential flow. This flow moves the solid-gas-suspension being formed above the annulus into rotation, thus producing a rotary fluidized bed.
With an appropriate selection of the velocities of both treatment gas currents, the rotating fluid bed can be stabilized and may contain a very high concentration of solid particles caused by centrifugal forces. As a consequence of this centrifugal force, which is a multiple of gravity force, the velocity of the treatment gas in the direction from outside to inside can be a multiple of the velocities inherent to conventional fluid beds.
This higher velocity generates better heat and mass transfer rates, both contributing together with the higher velocities to considerably smaller equipment sizes.
In order to forward such particles that have been carried over and deposited toward the center and are therefore removed from the centrifugal field of the flowing gas, the bottom forming the annulus is preferably designed as a rotating plate, so that deposited particles are hurled outside by a centrifugal force which is mechanically generated.
If the plate rotates with higher speed than the fluid bed, an eddy is formed because there are no breaking frictional forces present. An adjustable flap or a pulsation of the treatment gas passing through the annulus allows a discharge of treated goods from the fluid bed downwards, while new goods can be constantly fed into the fluid bed from outside.
A further step of the invention allows the immediate further treatment of the goods after its first treatment in a second, similiar or conventional fluid bed, effecting a genuine counter-current process. Conventional counter-current fluid bed processes have proved successful in exceptional cases only, due to the high susceptibility of clogging in the distributor bottoms. In the invented process, where fluid beds are built-up above relatively wide annuluses, the danger of clogging is overcome.